IB/hfi1: NULL pointer dereference when freeing rhashtable

A NULL pointer dereference occurs when the driver
is unloaded, and the SDMA rhashtable is freed if
the rhashtable_init() function has not been called.
Prevent this by changing sdma_rht to be a pointer
to a dynamically allocated hash table. The NULL-ness
of the pointer serves as an indication that the hash
table was initialized and that it needs to be
destroyed.

Fixes: 0cb2aa69 ("IB/hfi1: Add sysfs interface for affinity setup")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>